In a power plant of this type, for example, as disclosed in West German Pat. No. 32 12 266, the cooling gas blowers are arranged outside on the shell of the pressure vessel and extend, in relation to the diameter of the pressure vessel, in the radial direction far to the outside. Similarly, the control and shutdown installation and the second shutdown installation, including absorber elements in the form of absorber rods for insertion from the bottom into the small reactors, are protruding far from the bottom of the pressure vessel. The same is true for the discharge tube, whereby the spherical fuel elements may be taken from the small reactor. Even though the rest of the structural elements or components, such as for example the core, the cooling gas conduits, support elements, heat shields, reflectors, steam generators and the accessories of the said components are arranged within the pressure vessel and may therefore be installed, removed or replaced from above, this known embodiment is hardly appropriate for the construction of a subterranean nuclear reactor installation. The aforementioned components, which are protruding laterally and at the bottom from the pressure vessel and are in part accessible from below only and require a suitably large installation space, so that the economy of a subterranean construction of a nuclear reactor plant becomes questionable.